The present invention relates to a film pressure sensor in which errors in pressure measurement to be caused by temperature changes are reduced or removed.
Conventionally, a film pressure sensor is attached, for example, to an automotive seating equipment between its cover and a seat cushion material such as urethane so as to sense whether a passenger is seated or not on the seating equipment using the same.
An example of such film pressure sensor 10 is shown in FIGS. 12 and 13. The film pressure sensor 10 is of a constitution in which two insulating films each having a predetermined circuit pattern and a pressure-sensitive layer formed on one side are laminated via an insulating layer such that the pressure-sensitive layers of them oppose each other.
More specifically, the insulating films 11a and 11b are soft insulating plastic films such as of PET. Circuit patterns 12a and 12b are also formed on the insulating films 11a and 11b respectively. Here, the circuit patterns 12a and 12b are formed on the insulating films 11a and 11b by applying thereto metal foils formed to have predetermined patterns or by printing a conductive coating such as a silver ink to form predetermined patterns on the insulating films 11a and 11b, respectively. Further, pressure-sensitive layers 13a and 13b are printed on the circuit patterns 12a and 12b in predetermined positions, respectively. The pressure-sensitive layers 13a and 13b each have a substantially circular shape and are made of a resin composition, for example, a mixture of a polyurethane resin, carbon and an inorganic filler.
The insulating layer 14 is an insulating film such as of PET, and an adhesive is applied to both sides of it. The insulating layer 14 is punched only at such a part as opposes the pressure-sensitive layers 13a and 13b. The insulating layer 14 is adapted to be interposed between the insulating films 11a and 11b. Here, the insulating layer can be formed on one insulating film. In this case, the insulating layer can be formed by applying an adhesive layer, by means of printing or the like, on one insulating film excluding the area opposing the pressure-sensitive layers.
The thus constituted film pressure sensor 10 is designed to carry out pressure sensing over the area of the pressure-sensitive layers 13a and 13b arranged to oppose each other with a predetermined clearance secured therebetween. That is, when pressure is applied in one direction indicated by the arrow P or Q shown in FIG. 13 or in two directions P and Q, one of or both of the insulating films 11a and 11b is or are deformed elastically to bring the pressure-sensitive layers 13a and 13b into contact with each other and to cause a change in the contact resistance between the pressure-sensitive layers 13a and 13b depending on the pressure level. This change in the contact resistance is electrically sensed through the circuit patterns 12a and 12b to measure the pressure to be applied to the film pressure sensor 10.
The insulating films 11a and 11b, the pressure-sensitive layers 13a and 13b, the circuit patterns 12a and 12b, the insulating layer 14, etc. constituting the film pressure sensor 10 are of materials which are liable to undergo great temperature-dependent changes in terms of electrical properties including electrical resistance and physical properties including hardness. Accordingly, the hardness of the insulating films 11a and 11b, the contact resistance between the pressure-sensitive layers 13a and 13b, and electrical resistance of the circuit patterns 12a and 12b change with a temperature change, so that the sensed pressure value varies depending on the temperature even if the same pressure is applied to the sensor. Thus, the pressure to be applied to the film pressure sensor 10 cannot be measured accurately.
This sometimes gives rise to a problem particularly when the film pressure sensor 10 is attached to an automotive seating equipment. More specifically, in a comparison of the daytime temperature in the summer and the night temperature in the winter in an automotive cabin in one and the same district, there are cases where there is a difference of about 50xc2x0 C. or more. Such differences in the automotive cabin temperature as described above can occur between countries or districts where automobiles are used. Accordingly, when the film pressure sensor 10 is used in an automotive seating equipment, it is difficult to carry out accurate pressure sensing due to such differences in the automotive cabin temperature.
It is an objective of the present invention to provide a film pressure sensor in which errors in pressure measurement attributed to temperature changes are reduced or removed.
In order to attain the above objective, the film pressure sensor according to the present invention is characterized in that it contains a temperature sensing area which is formed on an insulating-film and senses the temperature of a pressure sensing area. The temperature sensing area is formed using a substance whose electrical resistance changes depending on the temperature.
Since the temperature sensing area measures accurately the temperature of the pressure sensing area, temperature-dependent errors in pressure measurement which are likely to be observed notably in film pressure sensors can be reduced or removed.
The film pressure sensor in one embodiment of the present invention is characterized in that a circuit pattern constituting the temperature sensing area is partly used commonly as a part of a circuit pattern constituting the pressure sensing area.
The number of circuit pattern terminals is reduced, and errors to be caused by temperature changes in the pressure measurement made by the film pressure sensor are reduced or removed by sensing the temperature of the pressure sensing area in the temperature sensing area.
In the film pressure sensor in another embodiment of the present invention, the temperature of the pressure sensing area is measured utilizing a change in an electrical resistance of the circuit pattern constituting the pressure sensing area caused by a temperature change.
Simpler circuit patterns are used to reduce the number of circuit pattern terminals and also to reduce or remove errors to be caused by temperature changes in the pressure measurement made by the film pressure sensor by sensing the temperature of the pressure sensing area.
The film pressure sensor in another embodiment of the present invention is characterized in that temperatures of the pressure sensing area and around this area are measured utilizing a change in the temperature-dependent electrical resistance of the temperature sensing area formed in the vicinity of a pressure-sensitive part of the pressure sensing area.
The temperature sensing area measures the temperature of the pressure-sensitive part that is influenced most significantly by temperature change and securely reduces or removes errors in pressure measurement.